Ultra-wideband (UWB) communication has attractive features for baseband multiple access, tactical wireless communications and multimedia services. In general, an UWB transmission consists of a train of very short pulses occupying an ultra-wide bandwidth. The information is typically encoded via either linear pulse amplitude modulation (PAM) or nonlinear pulse position modulation (PPM). The ultra-wide bandwidth includes bandwidths that are licensed from the Federal Communication Commission (FCC) for other communication purposes. However, the short pulses of the UWB transmission appear as minimal noise to non UWB-systems operating within those licensed frequencies.
Conveying information with ultra-short pulses can cause UWB transmissions to resolve many paths and become rich in multipath diversity. Consequently, rake receivers have been designed to collect the available multipath diversity to enhance the performance of UWB communication systems. Since the received UWB waveform often contains many delayed and scaled replicas of the transmitted pulses, a large number of fingers are typically needed on the Rake receiver. However, the Rake receiver may not have enough separation within the spectrum to allow each finger to track a different path of the transmission. At some point, the number of fingers on the Rake receiver may become too dense, and the receivers may become dependant on each other. In that case, no more additional diversity may be gained even if the number of fingers on the Rake receiver increases.
Moreover, each of the resolvable multipath waveforms undergoes a different channel, which causes distortion in the received pulse shapes. In some situations, the Rake receiver must know certain characteristics of each channel in order to correlate the received waveform with the delayed and scaled replicas. As a result, both the design and the implementation of Rake reception for UWB devices can be complicated. Furthermore, UWB transmissions have been shown to be very sensitive to timing jitter in non-fading channels. UWB transmissions with Rake reception are particularly sensitive to mistiming even in multipath fading channels.